Crystallographically-dependent ripple formation on Sn surface irradiated with focused ion beam

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1 Applied Surface Science 240 (2005) Crystallographically-dependent ripple formation on Sn surface irradiated with focused ion beam H.X. Qian a, W. Zhou a, *, Y.Q. Fu a, B.K.A. Ngoi a, G.C. Lim b a School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore , Singapore Received in revised form 19 May 2004; accepted 15 June 2004 Available online 19 August 2004 Abstract The metallographically polished polycrystalline Sn surface was sputtered by 30 kv focused Ga + ions at room temperature. The experiment was carried out using various FIB incidence angles (08, 158, 308, and 458) over a wide range of doses ( ions/cm 2 ). The surface morphology was carefully characterized under the optical microscope, scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated areas even at the normal FIB incidence angle, which is not consistent with the Bradley Harper (BH) rippling model. The orientation of ripples relies on crystallographic orientation rather than projected ion beam direction as predicted by BH model. The ripple wavelength is independent of ion dose, while ripple amplitude increases with ion dose. It is found that the ripples are formed by selforganization due to anisotropic surface diffusion in the low melting point metal. # 2004 Elsevier B.V. All rights reserved. PACS: Rf; Fx; Cf Keywords: Sn; Focused ion beam; Ripple; Crystallographic orientation; AFM; Surface diffusion 1. Introduction It is known that periodic height modulations can develop after off-normal incidence ion bombardment. Ripples have been observed on surfaces of a wide variety of materials such as glass [1], SiO 2 [2], fused * Corresponding author. Tel.: ; fax: address: wzhou@cantab.net (W. Zhou). silica [3], Ge[4], Si[5], and GaN [6] after they are irradiated with ions at an off-normal angle. In contrast, hillocks or depressions usually occur during ion sputtering at normal incidence angle [7]. According to the well-accepted Bradley Harper model (BH model) [8], two competitive forces determine the surface patterns: surface curvature dependent sputtering and smoothening due to surface diffusion. Within this linear model the ripple orientation depends on the ion beam direction, either perpendicular or parallel to the /$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 projected ion beam direction. This linear model was developed for amorphous or amorphized surfaces, and it can also explain many experimental results on crystalline materials. However, the model disregards the anisotropic diffusion effect on ripple formation. Ion sputtering induced rippling due to anisotropic diffusion has been observed on some single crystalline metals, such as Cu [9,10], Au[11] and Ag [12,13]. This work will give a detailed investigation of crystallographically-dependent anisotropic surface diffusion on ripple formation on polycrystalline Sn surfaces both at normal and off-normal incidence angles. H.X. Qian et al. / Applied Surface Science 240 (2005) Experimental The material used in the study was polycrystalline tin (purity >99.99%). The sample surfaces were metallographically polished using alumina suspension and then irradiated at room temperature with 30 kv Ga + ion using the Micrion dual beam FIB system (model 9500EX). The current was fixed at 1 na. The ion beam spot size was 60 nm in diameter at full width and half maximum (FWHM) at this current level. The beam spot overlap was 67% and the distance between two adjacent FIB spots was around 20 nm. Various FIB incidence angles (08, 158, 308, and 458) were used to study the effect of incidence angle on ripple formation, and sputtering time was varied to investigate the evolution of surface morphology over a wide range of doses (10 16 to ions/ cm 2 ). The irradiated surfaces were examined in air using the optical microscope and atomic force microscope (AFM) (model Nanoscope IIIa) and in vacuum using scanning electron microscope (SEM). Tapping mode was used in all the AFM measurements. 3. Results and discussion 3.1. Ripple formation at normal FIB incidence angle The FIB irradiation of tin surface at normal incidence angle is found to produce periodic ripples rather than depressions or hillocks. The ripples are even observable under the optical microscope, as shown in Fig. 1. It should be pointed out that the ripple formation cannot be attributed to direct machining by the focused ion beam for the following two reasons: Fig. 1. Surface morphology showing ripples formed at normal incidence angle in different grains. (a) Optical micrograph of ripples formed in one grain. (b) AFM image of ripples formed in another grain. The arrow indicates fast scan direction. first, the ripple wavelength (in the order of magnitude of 1000 nm) is far greater than the distance between two adjacent FIB spots (around 20 nm). Second, for the same fast scan direction, ripples formed in different grains are oriented in different directions. Fig. 1(a) shows that the ripple orientation is aligned in the fast scan direction, but Fig. 1(b) serves as an example to show that the ripple direction is neither parallel nor perpendicular to the fast scan direction. Since the ripples are not formed by direct FIB machining, they can only be formed by self-organization.

3 142 H.X. Qian et al. / Applied Surface Science 240 (2005) Spontaneous formation of ripples on ion sputtered surfaces is usually caused by a competition between ion erosion and surface diffusion. Ion erosion does not select any preferential direction at normal incidence angle, compared with that at off-normal ion sputtering. Surface diffusion plays a more important role in pattern formation. The observed formation of ripples at normal incidence angle indicates the existence of anisotropic surface diffusion. Diffusion occurs preferentially along the energetically favoured crystal orientation. Crystal structure of Sn is body centred tetragonal. The anisotropic diffusion biased by Ehrlich Schwoebel barriers [14,15] leads to the formation of ripple-like structure Ripple formation at off-normal FIB incidence angle Ripples were also observed on the Sn surface at various off-normal incidence angles. Typical ripples obtained at 158, 308 and 458 incidence angles are shown in Fig. 2. In contrast to the prediction by the BH model, the ripple orientation is not perpendicular to the projected ion beam direction. The ripple orien- Fig. 2. AFM images of the ripples formed at various incident angles and similar ion dose levels. (a) 158 and ions/cm 2. (b) 308 and ions/cm 2. (c) 458 and ions/cm 2. Note that the ripples are neither perpendicular nor parallel to the projected beam direction. The irradiated areas in (a) and (b) are within the same grain, but the irradiated area in (c) is located in a different grain. The curved arrow indicates the fast scan direction, while the straight arrow indicates the projected ion beam direction.

4 H.X. Qian et al. / Applied Surface Science 240 (2005) tation is always the same within any single grain but changes from one grain to another in the same experimental condition. The observations lend further support to the argument that formation of the ripples is due to crystallographically-dependent anisotropic diffusion during the ion sputtering process Effect of crystallographic orientation on ripple formation It was found that backscattered electron imaging and optical imaging can reveal grain boundaries of the polycrystalline Sn samples, making it possible to study effect of crystallographic orientation on ripple formation. As shown in Fig. 3, ripples of the same orientation formed in one of the grains but not in the adjacent grain, indicating clearly that the ripple formation is affected by the crystallographic orientation. In order to verify further that the crystallographic orientation rather than ion beam direction determines the ripple orientation, irradiated areas are rotated by 308 and 908 from the original position and rastered with focused ion beam under same conditions, respectively. The three pockets shown in Fig. 4(a c) are located in the same grain (i.e., within the same crystal), and again the ripples formed in the different Fig. 3. (a) Backscattered electron image showing a grain boundary and pockets irradiated at 308 incidence angle. (b) Optical image showing the same grain boundary and pockets in the same area as in (a). (c) Close-up of the pocket across the grain boundary identified in (a) and (b). Ripples can be seen in the pockets located in the left grain but not in the right grain. The curved arrow indicates the fast scan direction, while the straight arrow indicates the projected ion beam direction.

5 144 H.X. Qian et al. / Applied Surface Science 240 (2005) Fig. 5. Ripple amplitudes measured in the same grain as a function of ion dose at 458 incidence angle. pockets are aligned in the same direction even though the projected ion beam direction and fast scan direction are different. This provides further evidence to support the argument that crystallographic orientation determines the ripple formation Effect of ion dose on ripple formation Fig. 4. Optical images of pockets irradiated at 458 ion incidence angle with different ion beam scan directions and different beam projection directions. (a) Original position. (b) Rotation of fast scan direction by 308. (d) Rotation of fast scan direction by 908. The curved arrow indicates the fast scan direction, while the straight arrow indicates the projected ion beam direction. The observed surface morphology was further studied in quantitative manner. The ripple amplitude was measured using the atomic force microscope and is shown in Fig. 5 as a function of ion dose. It can be seen from the figure that the amplitude increases roughly linearly with increasing ion dose for the whole range of ion does studied ( to ions/cm 2 ). It should be noted, however, that ripple amplitude saturation has been observed by others for very high ion dose (around ions/cm 2 ), e.g., by Erlebacher et al. [16] on Si surface. They found that ripple amplitude will saturate after initial short period of exponential increase. Therefore, the increase of the ripple amplitude in Fig. 5 may be just in the first stage and amplitude under prolonged sputtering time need to be studied further. The relationship between wavelength and ion dose at 308 and 458 incidence angles is shown in Fig. 6. It can be seen from the figure that the wavelength is not sensitive to the change of ion dose in the dose range studied. It should be noticed that crystal orientation also has influence on ripple wavelength. Ripple wavelength at 308 in one grain is different form that at 458 in another grain.

6 H.X. Qian et al. / Applied Surface Science 240 (2005) is discussed that the ripple formation is due to crystallographically-dependent anisotropic surface diffusion. Further work needs to be carried out to study the quantitative relationship between the crystallographic orientation and the ripple orientation. The study will make it possible to choose certain crystallographic orientation and ion dose to obtain ripples with desired orientation, wavelength, and amplitude for certain applications. References Fig. 6. Ripple wavelengths measured in the same grain for 308 incidence angle and in another grain for 458 incidence angle and shown as a function of ion dose. In many previous studies, ripple wavelength is found to be independent of ion dose [17]. However, it is also reported that wavelength increases with ion dose following a scaling law l / t z (t is sputtering time or ion dose, and z is material-dependent numerical constant) [18,19]. The experimental results in the present study are in agreement with the dose-independent wavelength. However, it should be born in mind that the observation is valid for the dose range above ions/cm 2. In the very early stage of ripple formation, the scaling law might be applicable. 4. Conclusions Sn surfaces irradiated with focused ion beam were studied. Ripples were found to form at normal as well as off-normal incidence angles. The orientation of ripples relies on crystallographic orientation rather than projected ion beam direction. The ripple wavelength is independent of dose for the ion dose range studied. However, the ripple amplitude increases with ion dose in the very early stage of ripple formation. The experimental results indicate that crystallographic orientation has great influence on the ripple formation and the orientation. Therefore, it [1] M. Navez, C. Sella, D. Chaperot, Comptes Rendus. 254 (1962) 240. [2] T.M. Mayer, E. Chason, A.J. Howard, J. Appl. Phys. 76 (1994) [3] D. Flamm, F. Frost, D. Hirsch, Appl. Surf. Sci. 179 (2001) 95. [4] E. Chason, T.M. Mayer, B.K. Kellerman, D.T. McIlroy, A.J. Howard, Phys. Rev. Lett. 72 (1994) [5] J.J. Vajo, R.E. Doty, E.-H. Cirlin, J. Vac. Sci. Technol. A 14 (1996) [6] M. Kanazawa, A. Takano, Y. Higashi, M. Suzuki, Y. Homma, Appl. Surf. Sci (2003) 152. [7] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, H.L. Hartnagel, Science 285 (1999) [8] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol. A 6 (1988) [9] M. Ritter, S. Stindtmann, M. Farle, K. Baberschke, Surf. Sci. 348 (1996) 243. [10] J. Naumann, J. Osing, A.J. Quinn, I.V. Shvets, Surf. Sci. 388 (1997) 212. [11] M.V.R. Murty, T. Curcic, A. Judy, Phys. Rev. B 60 (1999) [12] S. Rusponi, C. Boragno, U. Valbusa, Phys. Rev. Lett. 78 (1997) [13] G. Costantini, S. Rusponi, F. Buatier de Mongeot, C. Boragno, U. Valbusa, J. Phys. Condens. Matter 13 (2001) [14] G. Ehrlich, F.G. Hudda, J. Chem. Phys. 44 ( ). [15] R.L. Schwoebel, E.J. Shipsey, J. Appl. Phys. 40 (1968) 614. [16] J. Erlebacher, M.J. Aziz, E. Chason, M.B. Sinclair, J.A. Floro, J. Vac. Sci. Technol. A 18 (2000) 115. [17] M.A. Makeev, R. Cuerno, A.-L. Barabási, Nucl. Instrum. Meth. B 197 (2002) 185. [18] S. Rusponi, G. Costantini, C. Boragno, U. Valbusa, Phys. Rev. Lett. 81 (1998) [19] S. Habenicht, W. Bolse, H. Feldermann, U. Geyer, H. Hofsäss, K.P. Lieb, F. Roccaforte, Europhys. Lett. 50 (2000) 209.